1. Field of the Invention
The present invention relates to a drawing method for optical fiber and a drawing furnace, and more particularly to a method for drawing an optical fiber while heat softening the optical fiber preform in a drawing furnace and to a drawing furnace used therefor.
2. Description of the Related Art
Compared to copper wire, optical fiber has the advantages of small size, light weight, low transmission loss, suitability for broadband transmission, etc. and has thus been increasingly adopted for telecommunication lines.
In conventional optical fiber manufacturing processes, normally a drawing furnace of the kind shown in FIG. 4 is used. Optical fiber preform 1 is inserted into furnace muffle tube 4 of drawing furnace 3, and the tip of preform 1 is melted by means of heater 6 which is positioned inside furnace body 5 and peripheral to furnace muffle tube 4. Optical fiber of the desired diameter is obtained by drawing the preform while applying a specified drawing tension by means of a capstan (not shown in the figure) and so on.
A variety of manufacturing methods and manufacturing equipments have been proposed for the manufacturing of optical fiber. For example, in Japanese Patent Application Laid-open No. Hei4-198036 a heating furnace having an annealing heater in its upper part has been disclosed, while in Japanese Utility Model Application Laid-open No. Sho61-147233 a heating furnace having a coil-shaped heater has been disclosed. In addition, in Japanese Patent Publication No. Hei6-2603, optical fiber manufacturing equipment having a anealing furnace between the drawing furnace and the coating equipment, and manufacturing equipment which uses that method have been disclosed and it is mentioned therein that defects in the optical fiber can be reduced by making the temperature distribution of the heat treatment furnace increase with proximity to the optical fiber preform. Furthermore, in Japanese Patent Publication No. Hei8-9490 a fiber drawing method has been disclosed wherein the specified conditions are achieved through the relationship between the length of the optical fiber preform and the line speed.
Recently demand has increased for optical fiber, such as dispersion compensation fiber, wherein the relative index difference (Δ+) of the center core compared to that of the cladding exceeds 1%. As means of achieving a higher deviation in the specific refraction index of optical fiber, the method of adding a germanium compound, such as germanium dioxide (GeO2), to the core has been proposed. In the core of optical fibers obtained in this way, a network of silicon atoms and germanium atoms bonded via oxygen atoms (Si—O—Ge) is formed.